The present invention relates to a bandpass filter, and particularly, to a highly compact bandpass filter that has excellent mechanical strength.
In recent years, marked advances in miniaturization of communication terminals, typically mobile phones, has been achieved thanks to miniaturization of the various components incorporated therein. One of the most important components incorporated in a communication terminal is a filter component.
As one type of filter component, Japanese Patent Laid Open No. 2000-68711 and Japanese Patent Laid Open No. 2000-183616, for example, each bandpass filters comprising a dielectric block formed with a plurality of holes whose inner walls are coated with metal plates. As another type of filter component, bandpass filters constituted by forming metal plates on irregular surfaces of a dielectric block are described in xe2x80x9cNovel Dielectric Waveguide Componentsxe2x80x94Microwave Applications of New Ceramic Materials (PROCEEDINGS OF THE IEEE, VOL.79, NO.6, JUNE 1991), p734, FIG. 31.xe2x80x9d
As a need continues to be felt for still further miniaturization of communication terminals such as mobile phones, further miniaturization of filter components, e.g., bandpass filters, incorporated therein is also required.
The mechanical strength of the above-mentioned types of filter components is, however, low because holes are formed in, or irregularities are formed on, the dielectric block constituting the main body. Miniaturization of the filter component is therefore impossible. Specifically, in the former type of filter component having holes formed in a dielectric block, mechanical strength of the dielectric block is low around the holes and in the latter type of filter component having irregularities formed on the surface of a dielectric block, mechanical strength is low around the recesses. Therefore, miniaturization of the filter component must be limited to ensure the mechanical strength at such portions.
Thus, in the prior art it is difficult to miniaturize filter components while ensuring sufficient mechanical strength. Therefore, a compact bandpass filter that has excellent mechanical strength is desired.
It is therefore an object of the present invention to provide a compact bandpass filter having excellent mechanical strength.
The above and other objects of the present invention can be accomplished by a bandpass filter comprising a dielectric block constituted of a first portion lying between a first cross-section of the dielectric block and a second cross-section of the dielectric block substantially parallel to the first cross-section and second and third portions divided by the first portion and metal plates formed on surfaces of the dielectric block, thereby enabling the first portion of the dielectric block and the metal plates formed thereon to act as an evanescent waveguide, the second portion of the dielectric block and the metal plates formed thereon to act as a first resonator, and the third portion of the dielectric block and the metal plates formed thereon to act as a second resonator, the metal plates including at least one exciting electrode formed on a first surface of the dielectric block, which has the widest area.
According to this aspect of the present invention, because the exciting electrode is formed on the first surface of the dielectric block, which has the widest area, a wide band characteristic can be obtained while using a very thin dielectric block. Further, when a very thin dielectric block is used, a high unloaded quality factor (Q0) can be obtained because the radiation loss is reduced.
In a preferred aspect of the present invention, substantially all of surfaces of the dielectric block substantially parallel to the first cross-section are open ends.
According to this preferred aspect of the present invention, because it is not necessary to form any metal plate or exciting electrode on the surfaces substantially parallel to the first cross-section, the fabrication cost can be reduced.
In a further preferred aspect of the present invention, the dielectric block has a substantially rectangular prismatic shape.
According to this preferred aspect of the present invention, because the dielectric block has a substantially rectangular prismatic shape, its mechanical strength becomes very high. Therefore, highly compact size and excellent mechanical strength can be obtained.
In a further preferred aspect of the present invention, exciting electrodes are formed on a corner or its adjacent region of the first surface of the dielectric block.
The above and other objects of the present invention can be also accomplished by a bandpass filter comprising:
a dielectric block having a top surface, a bottom surface, first and second side surfaces opposite to each other and third and fourth side surfaces opposite to each other, the dielectric block being constituted of a first portion lying between a first cross-section of the dielectric block substantially parallel to the first side surface and a second cross-section of the dielectric block substantially parallel to the first cross-section, a second portion lying between the first side surface and the first cross-section, and a third portion lying between the second side surface and the second cross-section;
a first metal plate formed on the top surface of the dielectric block corresponding to the second portion;
a second metal plate formed on the top surface of the dielectric block corresponding to the third portion;
a third metal plate formed on the third side surface of the dielectric block corresponding to the second portion;
a fourth metal plate formed on the third side surface of the dielectric block corresponding to the third portion;
a fifth metal plate formed on the bottom surface of the dielectric block;
a first exciting electrode formed on the bottom surface of the dielectric block corresponding to the second portion; and
a second exciting electrode formed on the bottom surface of the dielectric block corresponding to the third portion.
According to this aspect of the present invention, because the exciting electrodes are formed on the bottom surface of the dielectric block, a wide band characteristic can be obtained by thinning the dielectric block.
In a preferred aspect of the present invention, substantially all of the first and second side surfaces of the dielectric block are open ends.
In a further preferred aspect of the present invention, the bandpass filter further comprises a third exciting electrode formed on the fourth side surface of the dielectric block corresponding to the second portion and a fourth exciting electrode formed on the fourth side surface of the dielectric block corresponding to the third portion, the first and third exciting electrodes being in contact with each other and the second and fourth exciting electrodes being in contact with each other.
According to this preferred aspect of the present invention, because the external coupling is enhanced, still wider bandwidth can be obtained and the radiation loss can be reduced.
In a further preferred aspect of the present invention, the bandpass filter further comprises a capacitive stub formed on the fourth side surface of the dielectric block corresponding to at least the second and third portions.
According to this preferred aspect of the present invention, the overall size of the bandpass filter can be reduced.
In a further preferred aspect of the present invention, the fifth metal plate is in contact with the capacitive stub.
According to this preferred aspect of the present invention, because the effect of the capacitive stub is enhanced, the overall size of the bandpass filter can be further reduced.
In a further preferred aspect of the present invention, substantially all of the fourth side surface of the dielectric block is an open end.
According to this preferred aspect of the present invention, because it is not necessary to form a metal plate on the fourth side surface of the dielectric block, the fabrication cost can be reduced.
In a further preferred aspect of the present invention, a portion of the fifth metal plate formed on the surface of the second portion of the dielectric block and another portion of the fifth metal plate formed on the surface of the third portion of the dielectric block have the same dimensions.
In a further preferred aspect of the present invention, the dielectric block has a substantially rectangular prismatic shape.
In a further preferred aspect of the present invention, the second portion of the dielectric block, the first metal plate, the third metal plate, and a portion of the fifth metal plate formed on the surface of the second portion of the dielectric block are enabled to act as a first quarter-wave dielectric resonator and the third portion of the dielectric block, the second metal plate, the fourth metal plate, and another portion of the fifth metal plate formed on the surface of the third portion of the dielectric block are enabled to act as a second quarter-wave dielectric resonator.
The above and other objects of the present invention can be also accomplished by a bandpass filter, comprising:
a plurality of quarter-wave dielectric resonators including at least first and second quarter-wave dielectric resonators located in line, each of which is constituted of metal plates formed on a first surface of a dielectric block, a second surface of the dielectric block opposite to the first surface, and a third surface of the dielectric block substantially perpendicular to the first surface;
an evanescent waveguide interposed between adjacent quarter-wave dielectric resonators;
a first exciting electrode formed on the second surface of a portion of the dielectric block corresponding to the first quarter-wave dielectric resonator; and
a second exciting electrode formed on the second surface of another portion of the dielectric block corresponding to the second quarter-wave dielectric resonator.
In a preferred aspect of the present invention, a direct coupling is provided between the first and second exciting electrodes.
In a further preferred aspect of the present invention, the bandpass filter is substantially a rectangular prism in overall shape.
In a further preferred aspect of the present invention, substantially all of surfaces of the dielectric block perpendicular to both the first and third surfaces are open ends.
In a further preferred aspect of the present invention, the bandpass filter further comprises a capacitive stub formed on a surface of the dielectric block opposite to the third surface.
The above and other objects and features of the present invention will become apparent from the following description made with reference to the accompanying drawings.